Fluids being produced from oil wells may comprise a mixture of fluid components like oil, water, gas, and sand etc., which are commonly referred to as fluid phases, where the percentage fraction of each respective fluid component may vary from one oil field to another, and also during the operational lifetime of a same oil well. It is for example common to have an increase in water content from oil wells towards the end of the production life time of an oil well.
Separation of fluid components may be necessary to be able to provide further processing of the oil and gas in a refinery for example. However, the requirement of providing separation is not always a specific requirement. Sometimes it is only necessary to separate water from the oil, either as a course separation process, or with a higher demand on separation results and/or efficiency.
There are some proposals for separators in known art providing separation of water from fluids being produced from oil wells. A common technique is to utilize the known fact that water has a higher density than oil, and therefore it is possible to use gravity as a separating force. A common known separator arrangement is using a large tank where the fluids from the oil well are kept still for a time period. During this time period the gravity forces separate the oil and water, and the water is piled up at the bottom of the tank from where it can be removed, and gas may be piled up at the top of the tank above the oil since gas has a lower density than oil and water. Sand may also be piled up in the bottom of the tank together with the water.
It is also known more active types of separators, for example the class of separators denoted cyclone separators. The patent application EP0266348 A1 from 17 Jun. 1985 disclose a cyclone separator comprising a separating chamber (1), (2) (3); at least one inlet (8), for introducing feed to be separated into the cyclone separator and at least two outlets (4), (10), for discharging material from the separating chamber. The separator comprises at least one generally circumferential slot (20), disposed in the wall of the separating chamber downstream of each inlet slot (20), leading to or communicating with an exit from the separating chamber.
The gravitational separator tank installations are rather large and can usually only be located at centrally located places on land. Especially, in connection with oil production from the sea bed this has been regarded as an unfavourable solution. The paten publication U.S. Pat. No. 8,002,121 with priority from 15 Nov. 2004, by Michel Berard et al., addresses this problem by arranging an in-line flow separator being installable on a sea bed comprising an uphill section of a tubeline connected to a well head, where a first liquid (oil) and a second denser liquid (water) may flow through the separator from the bottom of the separator to an uphill located outlet opening. The second liquid forms a sump due to gravitation extending uphill from the bottom of the separator to an interface layer between the water and oil. An outlet in the bottom of the separator makes it possible to remove the water collected in the sump. A series of sensors are arranged in the uphill section to monitor the position of the interface layer between the water and the oil.
The international patent application WO 02/01044 by Skovholt et al. discloses an inclined gravitational separator having an inner tube arranged inside an outer tube. A tube carrying fluids from an oil well head are arranged to be in fluid communication with the inner tube via a tube joint located for example in a bottom surface of the inclined separator. The inner tube has a plurality of perforations (or holes) in the inner tube walls, and due to gravity water (and sand if present) in the fluid from the well head will fall through these openings, and will be collected in a sump in the bottom of the outer tube. The separated oil (and/or gas) will flow out of the inner tube from an opening in a top section of the separator. The pressure from the oil well drives the fluids through the separator system.
However, the plurality of openings, for example round holes, has a tendency to induce turbulence in the streaming flow out from the inner tube into the outer tube which may provide a slowing down of the flow of fluids and which may also provide an unfavourable mixing of fluid components, especially in the interface layer between water and oil. This condition can be difficult to handle if the fluid from the well head is under high pressure. Further, if the speed of the flow of fluid components in the tube is too high, the streaming of fluids past openings in the wall of the inner tube may provide a suction force due to the Venturi effect, as known to a person skilled in the art. Therefore, it is possible that the separator under certain conditions may extract (pump) water from the sump into the stream of fluids in the inner tube instead of separating for example the water from the oil. Therefore, it may be necessary to reduce the velocity of fluid streaming in the arrangement. According to the teaching of Skovholt et al. this can be achieved with valves that are controlled in a feedback loop, wherein control signals are generated proportional to signals from respective pressure transducers located in the separator, for example.
It may further be a problem that the interface layer between the water and oil in the separator may be a diffuse interface layer (no distinct border line between the fluids) and also that the interface layer may be located at different levels relative to for example the bottom of the separator. If the interface is too close to the bottom the number of openings in the inner tube walls the water may fall through due to gravitation will be less compared to a situation where the interface is higher up in the inclined separator. Therefore, the separation capacity may be variable due to for example variable flow rates.
There are also other further issues to take into consideration. For example slugs which are large volumes of liquid or gas that may appear as a unit out of different reasons and conditions in the pipelines transporting crude oil from oil wells. In known art it is common to arrange a slugcatcher that is located at the end of a pipeline and acts as a buffer to protect processing equipment. Slugs may move with higher velocities than expected fluids streaming from the oil field. The extra energy provided for by this increased velocity may then result in induced mechanical stress and shaking of the installation as known to a person skilled in the art.
Another problem with some crude oils is how water in the crude oil have impact on waxy crude oil gelation and rheology. These effects can provide obstacles to the streaming of the oil through a separator system, due to non-Newtonian behaviour of the fluids as well as gel clots that can block openings in the separator systems.
Hence, an improved oil separator would be advantageous, and in particular a more efficient and/or configurable and adaptable oil separator would be advantageous.